Santa Catalina Island, located ∼35 km off the Southern California coast, is home to the Blue Cavern Onshore State Marine Conservation Area (SMCA). Although the conservation area is recognized as both an area of special biological significance and a marine life refuge, species richness of the nearshore taxa from this location is not known. In this study, we provide a comprehensive inventory of the intertidal and subtidal marine macroalgae, plants, invertebrates, and fishes documented from 5 different reef sites inside Blue Cavern Onshore SMCA. Species richness data were compiled using scuba-based visual surveys conducted in the field, references from the primary and gray literature, research collections maintained by scholarly institutions, and field monitoring programs. The total number of marine taxa documented in this study (765 species from 17 major phylogenetic groups) represents 63% of the estimated species richness in Blue Cavern Onshore SMCA and is indicative of the high biodiversity known from this region. Specifically, the intertidal and subtidal biota reported here represent 34% and 18% of the marine taxa known from Catalina Island and the Southern California Bight, respectively. Incidences of the introduction of exotic and invasive organisms (n = 18), changes in the geographic distributions of species (n = 14), as well as marine taxa listed as species of concern, endangered, or critically endangered (n = 4), were also identified in the current inventory. Research findings presented here offer an important baseline of species richness in the California Channel Islands and will help to improve efforts by resource managers and policy makers to conserve and manage similar habitats in the coastal waters off Southern California.